Queen-worker ratio affects reproductive skew in a socially polymorphic ant

The partitioning of reproduction among individuals in communally breeding animals varies greatly among species, from the monopolization of reproduction (high reproductive skew) to similar contribution to the offspring in others (low skew). Reproductive skew models explain how relatedness or ecological constraints affect the magnitude of reproductive skew. They typically assume that individuals are capable of flexibly reacting to social and environmental changes. Most models predict a decrease of skew when benefits of staying in the group are reduced. In the ant Leptothorax acervorum, queens in colonies from marginal habitats form dominance hierarchies and only the top-ranking queen lays eggs (“functional monogyny”). In contrast, queens in colonies from extended coniferous forests throughout the Palaearctic rarely interact aggressively and all lay eggs (“polygyny”). An experimental increase of queen:worker ratios in colonies from low-skew populations elicits queen–queen aggression similar to that in functionally monogynous populations. Here, we show that this manipulation also results in increased reproductive inequalities among queens. Queens from natural overwintering colonies differed in the number of developing oocytes in their ovaries. These differences were greatly augmented in queens from colonies with increased queen:worker ratios relative to colonies with a low queen:worker ratio. As assumed by models of reproductive skew, L. acervorum colonies thus appear to be capable of flexibly adjusting reproductive skew to social conditions, yet in the opposite way than predicted by most models.

合作繁殖动物（communal breeding animals）不同物种间，个体间的繁殖分配模式差异显著：从繁殖权完全被单个个体垄断的高繁殖偏倚（reproductive skew），到多数个体对后代贡献均等的低繁殖偏倚。繁殖偏倚模型（reproductive skew models）阐释了亲缘关系或生态限制如何影响繁殖偏倚的程度。这类模型通常假定个体能够灵活响应社会与环境变化。多数模型预测，当留在种群中的收益降低时，繁殖偏倚会减弱。在铺道蚁（Leptothorax acervorum）中，来自边缘生境的蚁群内的蚁后会形成优势等级，仅最高等级的蚁后负责产卵，即功能单后制（functional monogyny）。与之相反，分布于全古北界广阔针叶林的蚁群内的蚁后极少发生攻击性互动，所有蚁后均可产卵，即多后制（polygyny）。研究人员对低繁殖偏倚种群的蚁群进行实验性提升蚁后与工蚁比例的操作后，蚁后间会出现与功能单后制种群类似的攻击行为。本研究表明，该操作同样会导致蚁后间的繁殖不均程度上升。自然越冬蚁群中的蚁后，其卵巢内发育中的卵母细胞数量本就存在差异。相较于蚁后与工蚁比例较低的蚁群，经提升蚁后与工蚁比例操作的蚁群中，蚁后的卵母细胞数量差异被显著放大。由此可见，该铺道蚁（L. acervorum）的蚁群能够像繁殖偏倚模型所假定的那样，根据社会环境灵活调整繁殖偏倚，但调整方向与多数模型的预测恰好相反。